This invention relates generally to magnetoresistive displacement or position measuring sensors, and more particularly, to a magnetoresistive sensor which is compensated to substantially reduce the influence of temperature variations on the accuracy of position measurements.
The use of magnetoresistor (MR) elements for sensing the position or displacement of a magnetically permeable member has been known for several years, see for example, U.S. Pat. No. 3,934,160 issued to Von Borcke in 1976. Such sensors typically include a pair of series connected MR elements forming a voltage divider, a permanent magnet for passing a magnetic flux through the MR elements, and a magnetically permeable member that moves proximate and relative to the MR elements to vary the amount of magnetic flux passing through the MR elements. The magnetically permeable member is generally configured to increase the magnetic flux passing through one MR element while decreasing the amount of flux passing through the other MR element as the member moves in one direction, and conversely when the member is moved in the opposite direction. As a consequence, the magnitude of the voltage appearing across the MR voltage divider provides an indication of the relative position of the magnetically permeable member, since the resistance of each MR element depends directly upon the amount of magnetic flux passing therethrough.
A principle problem associated with the above described type of magnetoresistive position sensors is their sensitivity to changes in operating temperature and the resulting decrease in measurement accuracy. Temperature compensation of such sensors is quite difficult because the temperature coefficient of resistance for the MR elements depends upon the strength of the magnetic field passing through the elements which varies with the position of the magnetically permeable member being measured.
The conventional approach for temperature compensating magnetoresistive position sensors has been to physically mount the two series connected magnetoresistors with two series connected thermistors in a bridge configuration within the same electronic package so the voltage exciting the MRs is adjusted with changing temperature. This traditional approach has several disadvantages. First, the temperature coefficients of resistance for the magnetoresistive elements and the thermistors can only be approximately matched in practice. Second, the physical interfacing of the thermistor and the MR circuitry prevents the thermistors and MRs from being at exactly the same temperature. Third, the mounting of thermistors adjacent to the MRs increase packaging size, cost, and number of required package interconnections.